Many payroll companies tax the payment as though that is the amount you always get. It avoids under withholding over the year. Unfortunately, it causes over withholding on your bonus payment.
On the bright side, you ought to find yourself getting a bunch of that money back on your tax return. I received almost 7k back due to that exact situation. It’s normal. Most people don’t notice because their bonuses are not substantial enough to be noticeable.
It's not because the payroll company withholds from it like you'd get it all year, it's because the IRS rules require bonuses have taxes withheld at a specific rate, and that rate (22% for most) may exceed your usual withholding
Hate to burst the bubble, but from what I understand:
If your highest tax bracket in 2018 is over 22%, you might not get a refund on a cent of the $5.2K for federal income tax you just paid in, unless you have other things going on that would reduce your tax liability. (Lots of itemized deductions, low earning spouse, capital losses, etc...)
There is a max OASDI (only taxed on first $128.4K of income), and if you hit it, they'll stop withholding, typically. So you won't see any of that back, but you may see them stop taking it once your income exceeds $128,400.
And there is no max to Medicare, so you won't see any of that, either.
I'm unfamiliar with NC state taxes so I can't comment on that.
Also, bonus withholding was 25% for years. I guess it is 22% now due to recent legislation. So it could be worse.